1. Field of the Invention
The present invention relates to a method for polymerizing alpha-olefins, a catalyst for such a polymerization method and a method for producing such a catalyst. In particular, the present invention relates to a catalyst, and a method for preparation thereof, which produces linear low density polyethylene (LLDPE) having narrow molecular weight distribution, as evidenced by relatively low values of melt flow ratio (MFR), and low hexane extractables, suitable for film and injection molding applications. The invention is also directed to a highly productive polymerization process carried out with the catalyst of the invention.
2. Description of the Prior Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as homopolymers of polyethylene. Certain of these properties are described in Anderson et al, U.S. Pat. No. 4,076,698.
Karol et al, U.S. Pat. No. 4,302,566, describe a process for producing linear low density polyethylene polymers in a gas phase, fluid bed reactor.
Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
Graff, U.S. Pat. No. 4,173,547, describes a supported catalyst obtained by treating a support with both an organoaluminum compound and an organomagnesium compound followed by contacting this treated support with a tetravalent titanium comound.
Stevens et al, U.S. Pat. No. 3,787,384, and Strobel et al, U.S. Pat. No. 4,148,754, describe catalysts prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound. According to the teachings of both of these patents, no unreacted organomagnesium compound is present when the reacted support is contacted with the tetravalent titanium compound.
Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W. L. Carrick et al in Journal of American Chemical Society, Volume 82, page 1502 (1060) and Volume 83, page 2654 (1961).
Nowlin et al, U.S. Pat. No. 4,481,301, disclose a supported alpha-olefin polymerization catalyst composition prepared by reacting a support containing OH groups with a stoichiometric excess of an organomagnesium composition, with respect to the OH groups content, and then reacting the product with a tetravalent titanium compound.
Dombro, U.S. Pat. Nos. 4,378,304 and 4,458,058, disclose an olefin polymerization catalyst composition synthesized by sequentially reacting: (1) a porous support with a Group IIA organometallic compound, e.g., a dialkylmagnesium; (2) the product of step (1) with water or a hydrocarbyl alcohol, e.g., methanol; (3) the product of step (2) with a transition metal compound or compounds. The product of the synthesis reaction is activated with a co-catalyst which is a Group IA, IIA, IIIA and/or IIB organometallic compound, including hydrogen. Suitable co-catalysts are n-butylithium, diethylmagnesium, triisobutylaluminum and diethylaluminum chloride.
Best, U.S. Pat. Nos. 4,558,024, 4,558,025 and 4,579,835, disclose olefin polymerization catalyst compositions prepared by reacting together a porous particulate material, an organic magnesium compound, an oxygen-containing compound, a transition metal compound, e.g., a titanium compound (the '024 patent) or a vanadium compound (the '835 patent), and a co-catalyst. Some of the catalyst compositions of Best also include an acyl halide (e.g., see the '835 and the '025 patents) and/or a Group IIIA hydrocarbyl dihalides, such as boron and aluminum alkyl dihalides (e.g., the '025 patent).
When the LLDPE resins are fabricated into injection-molded products, it is imperative to assure that such products are not susceptible to warping or shrinking. As is known to those skilled in the art, the degree of warping or shrinking can be predicted from the molecular weight distribution of the resins. Resins having relatively narrow molecular weight distribution produce injection-molded products exhibiting a minimum amount of warping or shrinkage. Conversely, resins having relatively broad molecular weight distribution produce injection-molded products more likely to undergo warping or shrinkage. One of the measures of the molecular weight distribution of the resin is melt flow ratio (MFR), which is the ratio of high melt flow index (HIMI or I.sub.21) to melt index (I.sub.2) for a given resin. The melt flow ratio is believed to be an indication of the molecular weight distribution of the polymer, the higher the value, the broader the molecular weight distribution. Resins having relatively low MFR values, e.g., of about 20 to about 50, have relatively narrow molecular weight distribution. Additionally, LLDPE resins having such relatively low MFR values produce films of better strength properties than resins with high MFR values. Many catalyst systems exhibit a tendency to produce resins whose MFR values, although initially low, increase with increased concentration of the catalyst activator, also known as a co-catalyst, such as various aluminum alkyls.
Another important property of LLDPE resins, manufactured into products coming into contact with articles subject to FDA regulations, e.g., foodstuffs, is hexane extractables which is a measure of the amount of low molecular weight and/or highly branched polymer molecules capable of being extracted from the manufactured products, e.g., plastic food containers, by hexane extraction. The FDA imposed strict regulations on the amounts of allowable hexane extractables in such plastic products.
Thus, Allen et al, European Patent Office (EPO) Application No. 87300536.1, published on Aug. 5, 1987, as publication No. 0231102, dislose an alpha-olefin polymerization catalyst composition activated with trimethylaluminum which produces polymers having relatively low values of MFR and low hexane extractables. However, the productivity of the polymerization process carried out with such a catalyst composition is lower than that of the process carried out with the same catalyst composition activated with more commonly-used activators, such as triethylaluminum and triisobutylaluminum.
Another important property of an alpha-olefin polymerization catalyst composition is the ability thereof to effectively copolymerize ethylene with higher alpha-olefins, e.g., C.sub.3 -C.sub.10 alpha-olefins, to produce resins having low densities. Such resins have important advantages, e.g., they are used to produce polyethylene film with excellent physical properties which is, therefore, substantially more resistant to tearing and puncturing than a film made from similar resins of higher densities. This property of the catalyst composition is referred to as "higher alpha-olefin incorporation property" and is usually measured by determining the amount of higher alpha-olefin (e.g., butene, hexene or octene) required in the polymerization process, e.g. fluid-bed reactor process, to produce a copolymer of ethylene and the higher alpha-olefin having a given density. The lesser is the amount of the higher alpha-olefin required to produce a resin of a given density, the higher are the production rates and, therefore, the lower is the cost of producing such a copolymer. Catalysts having good higher .alpha.-olefin incorporation properties are referred to in the art as having a high .alpha.-olefin incorporation factor. High values of the high .alpha.-olefin incorporation factor are especially important in the gas-phase fluid bed process, because relatively high concentrations of higher .alpha.-olefin in the fluid-bed reactor may cause poor fluidization caused, e.g., by resin stickiness. Therefore, production rates must be significantly reduced to avoid such problems. Consequently, catalyst compositions with a relatively high .alpha.-olefin incorporation factor values avoid these problems and are more desirable.
Accordingly, it is important to provide a catalyst composition capable of producing alpha-olefin polymers and copolymers having relatively narrow molecular weight distribution (low MFR values) and low densities.
It is therefore a primary object of the present invention to provide a high activity catalyst for the polymerization of alpha-olefins yielding products of a relatively narrow molecular weight distribution which is maintained substantially constant with varying amounts of the co-catalyst concentration.
It is another object of the present invention to provide a high activity catalyst composition which produces alpha-olefin polymers having relatively low hexane extractables.
It is yet another object of this invention to provide a high activity catalyst composition which has excellent higher alpha-olefin incorporation properties.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields linear low density polyethylene of a relatively narrow molecular weight distribution at high productivity rates.